Grain oriented electromagnetic steel sheets are widely used as iron cores of transformers, motors, and the like. These materials have crystal orientations highly accumulated in {110}<001>orientation referred to as “Goss orientation”, and the properties thereof are mainly evaluated by electromagnetic properties such as magnetic permeability, iron loss, etc.
In the process for producing a grain oriented electromagnetic steel sheet, an undercoating (glass coating) mainly composed of forsterite (Mg2SiO4) is generally formed on the surface thereof and suitably used as an insulating film and tension applying film. However, this film has the following problems.
In using a grain oriented electromagnetic steel sheet for an iron core of a transformer, a motor, or the like, the steel sheet must be processed into a predetermined shape by punching or shearing. Therefore, the grain oriented electromagnetic steel sheet is required to have the above electromagnetic properties and good processability. Particularly, a small-sized iron core called an EI core used for a power supply adapter, a fluorescent lamp, and the like comprises many laminated steel sheets, and thus punching quality of the electromagnetic steel sheet is an important problem which determines productivity of EI cores in mass production thereof.
The EI core will be described in detail below. FIG. 1 shows an example of the shape of the EI core. The EI core is produced by punching, but an effective processing method producing only a small amount of scrap in punching is used.
As an iron core material for the EI core, both a non-oriented electromagnetic steel sheet and a grain oriented electromagnetic steel sheet are used at present.
The grain oriented electromagnetic steel sheet has good magnetic properties in the rolling direction, but has much interior magnetic properties in the direction perpendicular to the rolling direction. However, in the EI core, a magnetic flux flows at an area ratio of about 20% in the direction perpendicular to the rolling direction, and flows at an area ratio of about 80% in the rolling direction. Therefore, when the grain oriented electromagnetic steel sheet is used as an ion core material of the EI core, much better properties can be obtained, as compared with the non-oriented electromagnetic steel sheet. Thus, the grain oriented electromagnetic steel sheet is used for many cases in which an iron loss is regarded as important.
As described above, the EI core is produced by punching a steel sheet using a die, but the forsterite undercoating is extremely harder than an organic resin film coated on the non-oriented electromagnetic steel sheet, thereby causing great abrasion of the punching die. Therefore, the die must be early re-polished or exchanged, causing deterioration in the working efficiency of core processing by a user and an increase in cost. Also, the presence of the forsterite undercoating deteriorates a slit property and cutting property.
The surface of the grain oriented electromagnetic steel sheet used for this purpose is required be free from the forsterite undercoating firstly, and many proposals have been made. An example of conceivable methods is a method in which a forsterite undercoating is formed, and then removed by pickling, chemical polishing, electropolishing, or the like. However, this method has a large problem in which the cost is increased, and the surface properties are worsened to deteriorate magnetic properties.
Recently, an attempt has been made to control the components of an annealing separator so as not to form a forsterite undercoating or decompose the forsterite undercoating immediately after the forsterite undercoating is formed, producing a grain oriented electromagnetic steel sheet having good processability.
For example, Japanese Unexamined Patent Application Publication No. 60-39123 discloses a method of inhibiting the production of a forsterite undercoating by using Al2O3 as a main component of an annealing separator. Also, Japanese Unexamined Patent Application Publication No. 6-17137 discloses a method of adding at least one of chlorides, carbonates, nitrates, sulfates and sulfides of Li, K, Na, Ba, Ca, Mg, Zn, Fe, Zr, Sn, Sr, Al, and the like to an annealing separator comprising MgO as a main component to decompose the formed forsterite undercoating. Furthermore, Japanese Unexamined Patent Application Publication No. 7-18333 discloses a method of removing a SiO2 undercoating formed in decarburization annealing by using an annealing separator containing 0.2% to 15% of Bi chloride and setting the nitrogen partial pressure of the final annealing atmosphere to 25% or more.
These means are capable of producing a grain oriented electromagnetic steel sheet without forming the forsterite undercoating. However, any one of these methods comprises the step of producing the forsterite undercoating or the oxide undercoating composed of SiO2 as a main component and then decomposing the undercoating, and requires a special releasing agent or auxiliary agent, thereby inevitably complicating the production process and causing the problem of increasing the cost.
For example, Japanese Examined Patent Application Publication No. 6-49948 and Japanese Examined Patent Application Publication No. 6-49949 propose a technique for suppressing the formation of a forsterite undercoating by mixing an agent with an annealing separator mainly composed of MgO and used for final annealing, and Japanese Unexamined Patent Application Publication No. 8-134542 proposes a technique for suppressing the formation of a forsterite undercoating by using an annealing separator mainly composed of silica and alumina for a material containing Mn. However, these methods can remove the adverse effect of the forsterite undercoating, but the problem of the coarse crystal grains of the grain oriented electromagnetic steel sheet is left unsolved.
Namely, the crystal grains of the grain oriented electromagnetic steel sheet are generally coarsened (usually about 10 to 50 mm) in the process of obtaining the strong Goss texture. Therefore, there is the problem of causing a large change in shape such as shear dropping or the like during punching, as compared with the non-oriented electromagnetic steel sheet generally comprising fine crystal grains of 0.03 to 0.20 mm. On the other hand, a usual method of suppressing the formation of coarse grains deteriorates the magnetic properties such as core loss, etc.
Therefore, means for satisfying both good punching ability and the magnetic properties such as core loss, etc. of the grain oriented electromagnetic steel sheet has not yet been established.
Furthermore, as described above, the grain oriented electromagnetic steel sheet has good magnetic properties in the rolling direction, but poor magnetic properties in the direction perpendicular to the rolling direction. Therefore, in application to the EI core in which a magnetic flux also flows in the direction perpendicular to the rolling direction, it is not said to make sufficient use of the properties of the grain oriented electromagnetic steel sheet.
For this problem, a method of developing a (100)<001> texture (regular cubic texture) by secondary recrystallization, i.e., a method of producing a so-called two-direction oriented electromagnetic steel sheet, has been investigated from old times.
For example, Japanese Examined Patent Application Publication No. 35-2657 discloses a method comprising performing cold rolling in one direction, performing cold rolling in a direction crossing the one direction to perform cross rolling, and then performing annealing for a short time and annealing at a high temperature of 900 to 1300° C. to obtain a strong cube texture in which regular cubic orientation grains are integrated by secondary recrystallization (using an inhibitor). Japanese Unexamined Patent Application Publication No. 4-362132 discloses a method comprising performing cold rolling with a rolling reduction of 50 to 90% in the direction perpendicular to the hot rolling direction, performing annealing for primary recrystallization, and then performing final annealing for secondary recrystallization and purification to secondarily recrystallize the regular cubic-orientation grains by using AlN.
Although a two-direction oriented electromagnetic steel sheet having good magnetic properties in both the rolling direction and the direction perpendicular to the rolling direction is most useful from the viewpoint of magnetic properties, cross rolling with very low productivity is required for producing the two-direction oriented electromagnetic steel sheet. Therefore, such a two-direction oriented electromagnetic steel sheet has not yet been put into industrial mass production.
Furthermore, in order to apply to the split core of a motor, Japanese Unexamined Patent Application Publication No. 2000-87139 discloses a technique of decreasing inhibitor components to develop the Goss orientation with a low degree of integration, decreasing anisotropy of the magnetic properties of the grain oriented electromagnetic steel sheet. However, this technique deteriorates the degree of integration of the Goss orientation and limits the Si amount to less than 3.0% by mass, and thus in an example, the iron loss W15/50 in the rolling direction is 2.1 W/kg or more, which is, at best, substantially the same as a high-quality non-oriented electromagnetic steel sheet, and is notably worse than the level of W15/50<1.4 W/kg of the grain oriented electromagnetic steel sheet. Therefore, this technique does not satisfy the requirements of users.
Apart from the above-described requirements, in some cases, iron core materials are required to exhibit a low iron loss in a high frequency region. Although whether or not this property is affected by the forsterite undercoating has not been known, the inventors found that a steel sheet without the forsterite undercoating developed by the inventors is very suitable for improving the high-frequency iron loss. Therefore, the technical background of this field is described here.
As a method of producing a grain oriented electromagnetic steel sheet having excellent high-frequency iron loss, Japanese Examined Patent Application Publication No. 7-42556 discloses a technique in which a grain oriented electromagnetic steel sheet having a highly developed Goss texture is used as a raw material, cold-rolled with a rolling reduction of 60 to 80% and then subjected to primary recrystallization annealing to obtain a product having a developed Goss texture and a thickness of 0.15 mm or less and comprising fine crystal grains having an average grain diameter of 1 mm or less.
However, this method comprises removing the forsterite undercoating from the grain oriented electromagnetic steel sheet, and performing rolling and recrystallization annealing, and thus this method costs much and is unsuitable for mass production.
Japanese Unexamined Patent Application Publication Nos. 64-5539, 2-57635, 7-76732 and 7-197126 disclose a method of producing a grain oriented electromagnetic steel thin sheet by using surface energy as a driving force without using an inhibitor.
However, there is a problem in which final annealing must be performed at a high temperature under conditions for suppressing the formation of a surface oxide in order to use the surface energy. For example, Japanese Unexamined Patent Application Publication No. 64-55339 discloses that a vacuum, an inert gas, a hydrogen gas, or a mixture of hydrogen gas and nitrogen gas must be used as an atmosphere of final annealing at a temperature of 1180° C. Japanese Unexamined Patent Application Publication No. 2-57635 recommends using an inert gas atmosphere, a hydrogen gas, or a mixed atmosphere of hydrogen gas and inert gas at a temperature of 950 to 1100° C. and further reducing the pressure of the gas. Furthermore, Japanese Unexamined Patent Application Publication No. 7-197126 discloses that final annealing is performed at a temperature of 1000 to 1300° C. in a non-oxidizing atmosphere at an oxygen partial pressure of 0.5 Pa or less or a vacuum.
As described above, in order to obtain good magnetic properties by using the surface energy, an inert gas or hydrogen is used as the atmosphere of final annealing, and a vacuum condition is required as a recommended condition. However, in view of equipment, it is very difficult to set both a high temperature and vacuum, thereby increasing the cost. When the surface energy is utilized, only the {110} plane can be basically selected, and growth of Goss grains in the <001> orientation coinciding with the rolling direction is not selected.
In the grain oriented electromagnetic steel sheet, the magnetic properties are improved by orienting the easy magnetization axis <001> in the rolling direction, and thus good magnetic properties are basically not obtained only by selecting the {110} plane.
Therefore, the rolling conditions and annealing conditions for obtaining good magnetic properties by a method using the surface energy are extremely limited, and thus the magnetic properties become unstable.
As described above, a method of obtaining a good high-frequency iron loss with a high cost efficiency has not yet been found.
As described above, the conventional techniques cannot produce a grain oriented electromagnetic steel sheet having good magnetic properties at low cost, and economically produce a grain oriented electromagnetic steel sheet having good punching quality without forming a forsterite undercoating on the surface.
In consideration of the above situation, it could be advantageous to provide a completely new grain oriented electromagnetic steel sheet excellent in processability and magnetic properties and economically advantageous, and a useful method of producing the same. The application of the steel sheet is not limited, but the steel sheet is ideally used as core materials of small-sized transformers, such as an EI core and the like.
It could also be advantageous to provide a grain oriented electromagnetic steel sheet further satisfying two-direction magnetic properties suitable for EI core materials, and a useful method of producing the steel sheet.
It could further be advantageous to provide a grain oriented electromagnetic steel sheet having highly developed Goss orientation and thus a high magnetic flux density, fine grains appropriately present in secondary recrystallized grains, and excellent iron loss in the high frequency region, and a useful method of producing the steel sheet.
In a process for producing a grain oriented electromagnetic steel sheet, inhibitor elements, for example, MnS, MnSc or AlN, are generally contained in a steel slab used as a starting raw material to selectively grow Goss orientation crystal grains. Therefore, in finish annealing, a so-called “purification annealing process,” i.e., annealing at a high temperature of 1200 to 1300° C. in a pure hydrogen stream, is required, and it is thus very difficult to avoid the problems of forming a coating, coarsening the grains and increasing the cost.
On the other hand, as a result of intensive research on the reason for secondary recrystallization of {110} <001> orientation grains, we found that grain boundaries having an orientation difference angle of 20 to 45° in a primary recrystallized structure play an important role, and reported this finding in Acta Material, Vol. 45 (1997), p.1285. This shows that the function of the inhibitor is to produce a “difference between the moving speeds of high-energy grain boundaries and other grain boundaries, and even if the inhibitor is not used, secondary recrystallization is allowed to take place by producing a difference between the moving speeds of the grain boundaries.
On the basis of this finding, we proposed a technique for developing Goss orientation crystal grains by secondary recrystallization of a material not containing the inhibitor component (Japanese Unexamined Patent Application Publication No. 2000-129356).
However, we sought further improvement and conducted intensive research for obtaining a grain oriented electromagnetic steel sheet suitable for small-sized electric apparatuses such as an EI core, in which punching processability is regarded as important.